This application is based on and claims priority under 35 U.S.C. xc2xa7 119 with respect to Japanese Application No. 2000-329153 filed on Oct. 27, 2000 the entire content of which is incorporated herein by reference.
The present invention generally relates to a shift control method for an automatic transmission in a vehicle. More specifically, the present invention pertains to a shift control method for an automatic transmission in which the shift pattern is changeable from a first shift pattern for selecting gears in response to a vehicle speed and a throttle valve opening degree to a second shift pattern for selecting gears in response to a deceleration of the vehicle or vice versa based on a vehicle running condition.
A known control method for an automatic transmission is disclosed in a Japanese Patent Application Publication published on May 16, 1995 as Toku-Kai-Hei 7 (1995)-127720. This publication discloses a shift control method for an automatic transmission for switching from a first shift pattern to a second shift pattern depending on the throttle opening degree, the shift range set by a shift lever, the gear stage, and the vehicle speed. The first shift pattern is normally used for controlling an automatic transmission to shift between gear stages based on the vehicle speed and the throttle valve opening degree. On the other hand, the second shift pattern is used to control the automatic transmission to shift between gear stages based on the gradient of road and the vehicle speed.
Generally speaking, the throttle valve opening degree is decreased when the driver releases the acceleration pedal while a vehicle is running. Then, the vehicle shifting condition is switched from the first shift pattern to the second shift pattern to be shifted to a lower speed gear stage. In the second shift pattern, when the road condition is descending, a shift line for setting to shift the gear stage is set at a higher speed side than when the road condition is flat so that engine braking (deceleration by resistance of the internal combustion engine) can be efficiently carried out. Accordingly, the engine braking tends to be performed excessively. The driver may depress the acceleration pedal to keep the vehicle speed or to accelerate immediately after the vehicle speed becomes smaller than an expected speed by the driver. Therefore, the vehicle shifting condition is switched from the second shift pattern to the first shift pattern. The driver will release the acceleration pedal again to close the throttle valve when the vehicle speed is increased excessively. Then the vehicle shifting condition is switched from the first shift pattern to the second shift pattern. In this way, under some road conditions, the driver repeatedly depresses and releases the acceleration pedal under the vehicle condition wherein the shift pattern is switched from the first shift pattern to the second shift pattern or vice versa. The repeated increasing and decreasing of the throttle valve opening degree causes a frequent switching between the first shift pattern and the second shift pattern, which may lead to hunting of the shifting gear stages.
In accordance with an aspect of this invention, a shift control method for an automatic transmission is adapted to shift a gear stage of the automatic transmission based on one of a first shift pattern in which gear stage shifting is based on vehicle speed and an opening degree of a throttle valve and a second shift pattern based on the vehicle speed and a road condition, with shifting of the gear stage being based on the second shift pattern when the vehicle driving condition is in a deceleration condition. The control method includes switching the vehicle shifting condition from the first shift pattern to the second shift pattern when the opening degree of the throttle valve is a first predetermined opening degree or is smaller than the first predetermined opening degree. The vehicle shifting condition is switched from the second shift pattern to the first shift pattern when the opening degree of the throttle valve is a second predetermined opening degree or larger than the second predetermined opening degree, with the second predetermined opening degree being larger than the first predetermined opening degree.
The vehicle shifting condition is inhibited from switching from the second shift pattern to the first shift pattern when an increasing rate of the opening degree of the throttle valve is larger than a predetermined increasing rate, even if the opening degree of the throttle valve is the second opening degree or is larger than the second opening degree. In addition, the operation of an exhaust brake is controlled when the opening degree of the throttle valve becomes the first predetermined opening degree or smaller than the first predetermined opening degree. The method further involves stopping the operation of the exhaust brake when the opening degree of the throttle valve becomes a third predetermined opening degree or larger than the third predetermined opening degree from a controlling condition of the exhaust brake, with the third predetermined opening degree being larger than the second predetermined opening degree.